The bassinet position in 20E is definitely only big enough for 1 cot, so if you want one for each twin, then you have to book 20E and 64A if its a mid J plane, or 14E and 20E if high J. Also on the mid J 20E is indeed just in front of WT.

With 64AK then you won't be able to let both sleep at the same time in a cot, unless you are happy cradling one of them to sleep. I think that its a bit crazy to go for UD if you have not flown with them before, they could be as good as gold but they may also scream the whole flight. You'll see there are plenty of threads of the topic of babies in the UD and that's partly why we went for 20EF when we flew to HKG. Even on this busy route, 20D was empty on the return leg so we made good use of it, whereas UD was full.